1. Technical field
The disclosure relates in general to Seebeck/Peltier effect thermoelectric conversion devices and in particular to devices using layers of conductive or semi conductive material deposited over a substrate even of large size by common planar techniques and with electrical contacts definable with noncritical lithographic or serigraphic techniques.
2. Reference Notions
The Seebeck effect is a thermoelectric phenomenon according to which a difference of temperature at the opposite ends of an elongated conductor or semiconductor generates electricity. The effect, discovered by the physicist Thomas J. Seebeck in 1821, manifests itself with the presence of a voltage difference at the two ends of a conductive bar subjected to a gradient of temperature ∇T. In a circuit including two junctions kept at different temperatures T1 and T2, between different materials A and B the voltage difference between the two junctions is given by:
                    V        =                              ∫                          T              1                                      T              2                                ⁢                                    [                                                                    S                    B                                    ⁡                                      (                    T                    )                                                  -                                                      S                    A                                    ⁡                                      (                    T                    )                                                              ]                        ⁢                                                  ⁢                          ⅆ              T                                                          (        1        )            
where: SA and SB are the Seebeck coefficients (also referred to as thermoelectric powers) relative to the two materials A and B. The voltage values are typically in the order of few μV/K. The Seebeck coefficients depend on the materials, on their absolute temperature and on their structure. The Seebeck effect may be exploited for making devices adapted to measure temperature differences, in terms of voltage differences in a circuit constituted by wires of different materials (thermocouple) or for generating electrical energy (thermopile) by connecting in series a certain number of thermocouples.
From a microscopic point of view, the charge carriers (electrons in metals, electrons and holes in semiconductors, ions in ionic conductors) diffuse when one end of the elongated conductor is at a temperature different from the temperature at the other end. The carriers at higher temperature will diffuse toward the zone at a lower temperature as long as there are different densities of carriers in the portion at lower temperature and in the portion at higher temperature of the elongated conductor. In an isolated system, equilibrium will be reached when, through a diffusion process, heat will become uniformly distributed along the whole conductor. Redistribution of thermal energy due to the movement of charge carriers implies a thermal current and of course such an electrical current will become null when the temperature of the system becomes uniform. In a system where two junctions are kept at a constant difference of temperature, also the thermal current will be constant and therefore a constant flux of charge carriers will be observed. Carrier mobility is reduced by scattering phenomena caused by impurities present in the lattice of the material, by structural defects and by lattice vibrations (phonons). Therefore, the Seebeck coefficient of a material depends in a significant measure on the density of impurities and of crystallographic defects further than on the material phonon spectrum. On the other end, phonons are not always locally in thermal equilibrium. On the contrary they move following the temperature gradient and loose energy by interacting with electrons or other carriers, as well as with the lattice defects. If the phonon-electron interaction is predominant, the phonons will tend to push electrons toward a portion of the elongated conductor loosing energy in the process, thus contributing to the electric field in the conductor film. These contributions are ever more important in the temperature range in which the phonon-electron scattering phenomenon is predominant, that is for
                    T        ≈                              1            5                    ⁢                      θ            D                                              (        2        )            
where θD is the Debye temperature. At temperatures lower than θD there are fewer phonons that are available for energy transfer while at temperatures above θD they tend to lose energy through a sequence of phonon-phonon collisions rather than through repeated phonon-electron collisions.
It is useful to define a thermoelectric factor of merit of a material as:
                    Z        =                              S            2                                κ            ⁢                                                  ⁢            ρ                                              (        3        )            
where κ and ρ are the heat conductivity and the electrical resistivity, respectively, of the material.
From a technological point of view, the use of Seebeck/Peltier effect thermoelectric converters has being considered for potentially important commercial application. More than half of the heat generated in a thermoelectric power plant is at present dissipated as low enthalpy heat. It is estimated that about 15 millions of megawatt are dispersed in the process of energy conversion alone. Availability of Seebeck generators capable of converting even only part of such amount of low enthalpy heat in electricity would have a significant positive impact on the energy shortage problem.
Known candidates as thermoelectrically active materials generators have a rather low factor of merit. For example, in case of a thin film of n silicon, doped with 5×1015 atoms of As per cm3, at room temperature, Z≈10−3 K−1. Values of ZT≈1 may be obtained only with costly materials of scarce availability such as Bi2Te3 or alloys of Sb or Se, as an example. In practice, besides few uses at relatively high added value, such as for thermoelectric generation in spacecrafts, the thermoelectric generators based on massive low cost materials achieve conversion yields of the thermal power to electrical power of just about 7%. By comparison, a turbine engine is capable of converting about 20% of the thermal energy to electrical energy.
In order to increase the factor of merit, the numerator of Eq. (3) should be maximized and/or the denominator should be minimized.
Analyzing the denominator, the formula may be written as:
      Z    =                            S          2                          ρ          ⁡                      (                                          κ                ph                            +                              κ                el                                      )                              =                        S          2                                      ρ            ⁢                                                  ⁢                          κ              ph                                +          LT                      where      L    =                            ρ          ⁢                                          ⁢                      κ            el                          T            =                                    π            2                    3                ⁢                              (                                          k                B                            q                        )                    2                    
and where −q is the electron charge and the suffixes ph and el indicate pertinence to phonons and to electrons, respectively.
According to Wiedemann-Franz Law, L is almost a universal constant of about 2.44×10−8 WΩK−2, because in metals the ratio between heat and electrical conductivities (σ=1/ρ) is almost the same at a given temperature T. For a good thermoelectric material κe1, that is LT/ρ, should always be much less than κph. That is to say that heat conduction should not be dominated by electrons.
Therefore, the doping of a semiconducting material destined to be used in Seebeck effect devices must be adapted to ensure a high electrical conductivity without significantly affecting heat conductivity.
Analyzing the numerator one has
  S  =                              k          B                q            ⁢              (                              5            2                    -                      ln            ⁡                          (                                                N                  d                                                  N                                      V                    -                    C                                                              )                                      )              =                            k          B                q            ⁢              (                              5            2                    +                      ln            ⁡                          (                              q                ⁢                                                                  ⁢                μ                ⁢                                                                  ⁢                ρ                ⁢                                                                  ⁢                                  N                                      V                    -                    C                                                              )                                      )            
where NV-C is the density of states in the appropriate band depending on the dopant used and Nd(=1/q,μρ) is the active dopant concentration.
Therefore, though S varies with resistivity, it does so logarithmically (that is at a much reduced rate).
3. Discussion of the Prior Art
Lately it has been shown [1, 2] how a system of drastically reduced size (nanowires of silicon with transversal dimensions in the order of 20 nm) and having suitably roughened surfaces may manifest a relatively high thermoelectric factor of merit. Enhancement of the Z factor derives from a “decoupling” between the mean free path figures of phonons and electrons caused by a significant scattering of phonons at the surface of the conductive nanowire. In particular, the important contribution to heat conductivity deriving from acoustic phonons of relatively lower frequency (longer wavelength) may be almost completely eliminated, being null in the material the density of phonons of wavelength greater than the cross section dimensions of the wire. As a consequence, the heat conductivity of silicon drops from ≈150 W m−1 K−1 (at room temperature for massive Si) to ≈1.6 W m−1 K−1 (at room temperature for nanowires of Si of 20 nm in cross section). Unfortunately, these test devices made with silicon nanowires are made with techniques unsuitable to industrialization on large scale.
In a prior published patent applications No. WO 2009/125317 and in prior Italian patent application No. VA2009A000082, filed on Dec. 15, 2009, of the same applicant, methods are described for making nanowires of elements belonging to the IV Group of the Periodic Table or of alloys thereof, without requiring the use of advanced lithographic techniques of definition, in the realm of few tens of nanometers, and with a great control of the surface roughness of the nanowires, adapted to modify the mean free paths of phonons and electrons by exploiting even cavity surfaces produced in a controlled manner within the bulk of the nanowires. The disclosed processes though much simpler than the fabrication processes previously used for making nanosized elongated structures, still requires lithographic processing, anisotropic etchings and conformal deposition processes in vacuum.
In prior Italian patent application No. VA2009A000050, filed on Jul. 15, 2009, of the same applicant, a conversion device is described made of a stack of dielectric layers alternated to semiconducting layers even of large area that after deposition are implanted with ions of a noble gas or with N, F, or O, at gradually varied kinetic energy and fluence, and successively subjected to a degassing treatment though thermal cycling for favoring gaseous molecular aggregations within the bulk of the semiconductor that cause formation of nanocavities, uniformly distributed in the material, before being eventually released off in part.
In prior Italian patent application No. VA2009A000050, filed on Jul. 15, 2009, of the same applicant, a conversion device is described made of a stack of dielectric layers alternated to semiconducting layers even of large area. In such bi-dimensional nanometric structure of heat-conductive material the heat-escape area, wh (with w and h width and thickness of such layers), is small enough to prevent direct escape paths across the planar surfaces. When this basic condition is coupled to that of making the escape surfaces not smooth and substantially parallel to the direction of the temperature gradient across a septum, but relatively rough that is with nanometric profile irregularities adapted to contrast migration of reflected phonons in the direction of the temperature gradient across a septum by enhanced net backward inelastic scattering at the surface, a remarkably incremented thermoelectric factor of merit is observed. Surface irregularities with mean peak-to-valley amplitude and mean periodicity comprised between 5 to 20 nm are outstandingly effective. By depositing the conductive material with intermittent air breaks or by depositing the dielectric material in form of a sub-oxide that is successively thermally decomposed to a mixture of stoichiometric dielectric oxide and semiconductor, the above discussed type of nanometric surface irregularities at conductor/dielectric interfaces are obtained.